Since the invention of power vehicles, many different powertrain systems have been attempted, including a steam engine with a boiler or an electric motor with a storage battery. It was, however, the discovery of petroleum in 1856 in the four-stroke internal combustion engine invented by Otto in 1876 that provided the impetus for the modern motor vehicle industry.
Although fossil fuel emerged as the fuel of choice for motor vehicles, recent concerns regarding fuel availability and increasingly stringent federal and state emission regulations have renewed interest in alternative fuel powered vehicles. For example, alternative fuel vehicles may be powered by methanol, ethanol, natural gas, electricity, or a combination of these fuels.
A dedicated electric powered vehicle offers several advantages: electricity is readily available, an electric powered distribution system is already in place, and an electric powered vehicle produces no emission. There are, however, several technological disadvantages that must be overcome before electric powered vehicles gain acceptance in the marketplace.
Hybrid powered vehicles, powered by both an internal combustion engine and an electric motor, have been widely proposed for overcoming the technical disadvantages of a dedicated electric vehicle while still offering an increased efficiency. The performance and range characteristics of a hybrid powered vehicle is comparable to a conventional fossil fuel powered vehicle. However, a great deal of development is still necessary in order to provide a hybrid electric vehicle which would be widely accepted by the consuming public.
One of the key contributors of electric and hybrid electric vehicle powertrains to the achievement of adequate driving range and fuel efficiency is the ability to regenerate braking energy for later use in propelling the vehicle rather than to reject it in the form of heat. Achieving repeatable and consistent braking performance in a vehicle with two mechanisms of braking, i.e., friction brakes and electric motor regeneration braking, and where the effect of the electric motor regenerative braking is necessarily variable and inadequate to do the total job, it is necessary to make the friction brake mechanism controllably variable so that the sum of the two braking mechanisms working together provides a level of braking performance consistent with the expectations of the driver. Traditional friction brake systems do not provide the desired controllable variability.
Recently developed regenerative braking systems often achieve the desired friction brake controllable variability by isolating the driver's brake pedal and foot from the function of the friction brakes by first simulating the traditional feel of the brakes at the pedal and second, providing the desired controllably variable friction braking by means of either computer controlled electrically actuated or servo-hydraulically actuated friction brakes.
The present invention takes a different approach in that it provides a means of superimposing on a traditional vacuum power hydraulic brake system a means of controllably reducing the energy introduced by the vehicle driver into the friction brake system by an amount that is a function of the vehicle braking provided by the electric motor in a regeneration mode.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.